Many automobile engines currently on the market utilize an endless power transmission belt for driving a plurality of driven accessories. They employ a tensioning system utilized to provide a tensioning force on the endless power transmission belt, which may be of any suitable type known in the art. Preferably, the belt is made primarily of a polymeric material because the unique features of the tensioner of this invention readily permit the tensioner to tension a belt having a polyester load-carrying cord in an efficient manner.
In many of these automotive accessory drives it is necessary to provide a correct tension to control a tension ratio throughout the life of the belt. With the advent of the single belt V-ribbed drive system, this is of increasing importance since belts are longer and some accessories are driven off the backside of the belt as a flat belt drive. Automatic tensioners of various descriptions have been developed having the requisite characteristics enabling them to tune the belt system to remove input torsionals and prevent or reduce harmonics, while allowing the tensioner to respond to changes in the belt tension requirements. For instance, see U.S. Pat. Nos. 4,596,538, 4,832,666, and 5,443,424 to Henderson, U.S. Pat. Nos. 4,938,734, 5,030,172 and 5,035,679 to Green, et. al., U.S. Pat. No. 5,190,502 to Gardner, et. al., or U.S. Pat. No. 5,348,514 to Foley, all now incorporated into this application by this reference thereto. A problem is that a torsion spring cannot be made with a rate characteristic to both resiliently tension a belt and prevent bubble or slack length from developing in the belt during periods of extreme engine deceleration, i.e., that allows for asymmetric damping.
For optimal function of a V-ribbed, flat belt, or V belt tensioner, it is desirable that the tensioner move easily and quickly toward the belt to take up slack, but provide more than the same resistance to prevent the belt from lifting the tensioner arm away from the belt. This feature is desirable for proper control of steady state accessory torque loads that are occasionally interrupted with a non-steady state or reverse transient load, such as a wide-open-throttle (WOT) one-two gearshift in manual and automatic transmissions. During WOT, the engine suddenly goes from, for example, 5000 RPM to 3500 RPM, which is similar to putting a brake on the engine. The current tensioner then becomes an untensioner, which can cause belt slip because the tensioner will be lifted off the belt by the high tension in what is normally the low tension side of the system, allowing extra belt length to occur on the opposite side of the system.
Asymmetric damping is also desirable to control engine start up transients due to slow combustion events and rapid engine acceleration during first firing. Further, this motion is desirable to control torque pulses of engines having lightweight flywheels or “dual mass” flywheels, where the combustion torque variation can exceed levels equal to the average accessory torque load at idle at the crankshaft driver pulley.
It is known to have asymmetric motion control using hydraulic linkage with directional fluid orifices as in U.S. Pat. No. 5,924,947 to Williams and U.S. Pat. No. 4,822,322 to Martin et. al.
It is also known to have asymmetric motion control using dry or lubricated surface friction, such as a brake band, as in U.S. Pat. No. 5,354,242 to St. John. This system is limited, however, in its ability to provide asymmetric motion by the amount of angular vector shift with a change in rotational direction and that requires excessive rotational motion to tighten the band in the high torque direction.
As taught by U.S. Pat. No. 5,935,032 to Bral, it is also known to have asymmetric motion control using damping friction surfaces that are limited in friction torque developed by the amount of normal load that can be generated by a spring and that need lots of angular displacement to engage and disengage, where the displacement is amplified by a conical wedging action.
It is also known to have asymmetric motion control using an “elastomer sandwich” that is severely limited in range of operation by the very steep spring rates of the compressed elastomers as in U.S. Pat. No. 5,171,188 to Lardrot. The tensioner taught by Lardrot, however, suffers from a lack of angular rigidity since its center of pivot floats, and thus is not absolutely controlled, for instance see.
The present embodiments overcome these deficiencies and may accomplish the above-discussed functions for asymmetric motion control, and can be applied to any conventional rotating tensioner that uses a rotational spring to rotate the tensioner arm toward the belt to create belt tension.